1. Field of the Invention
The present invention relates to methods and apparatus for creating video display signals suitable for generating a video display of a borehole and more particularly to such methods and apparatus in which the display signals are derived from information generated by acoustically scanning a reflecting boundary circumferential of the borehole.
2. Setting of the Invention
One type of device for logging a borehole to determine the characteristics of the surrounding formation is sometimes referred to as a borehole televiewer. Such a device includes a motor received a housing which is receivable in a wellbore. A transducer assembly is received in the housing and may be rotated by the motor. As the housing is raised or lowered in a wellbore, power is supplied to the motor via a cable which connects the housing to a generator at the surface of the wellbore. Periodic transducer firing pulses are applied to the rotating transducer assembly. Such firing pulses cause the transducer assembly to emit an acoustic pulse along a radial borehole axis for each firing pulse applied thereto. When periodic firing pulses are applied to the transducer assembly, the borehole is helically scanned with acoustic pulses. A portion of the energy from each acoustic pulse is reflected by a reflecting boundary adjacent the borehole along a radial axis back toward the transducer assembly which detects the reflected energy. The reflecting boundary may be the radially inner surface of the casing, the casing-borehole interface or an interface between formations in the ground surrounding the wellbore. If logging is conducted in an uncased well, the borehole wall comprises one reflecting boundary.
An electrical pulse is generated which is related to the energy of the reflected acoustic pulse detected by the transducer. Information regarding the relative times at which the firing pulses and the electrical pulses occur and the magnitude of the electrical pulses is transmitted to the surface on a cable. This information can be used to generate a video display of the borehole wall which the tool has scanned.
One method and apparatus for creating such a video display is disclosed in U.S. Pat. No. 3,728,672 to Dennis et al. In Dennis et al., the electrical pulses generated by reflected acoustic pulses in the borehole are used to modulate the z-axis of a video monitor while a horizontal or x-axis sweep is initiated for each full revolution of the transducer. Thus, a new sweep of the video monitor is initiated for each 360.degree. scan of the borehole wall and an image relating to the characteristics of the borehole wall appears on the monitor.
U.S. Pat. No. 4,463,378 to Rambow discloses a borehole televiewer display which creates a video display of a borehole in a somewhat different manner than that disclosed in the Dennis et al., patent. In Rambow, the peak amplitude of each electrical signal is converted to a digital number which is supplied to a conventional computer that is programmed to arrange the digitized amplitude signals in the same order in which they are received. The signals corresponding to one cycle or scan of the tool are arranged to provide one horizontal sweep of a television monitor and each signal is assigned a shade of gray depending upon its amplitude. The computer also stores in its memory the number of cycles corresponding to the number of lines for a complete video display, normally 512 lines. The computer continually replaces the oldest cycle with a new cycle in its memory and retains only the 512 lines.
In another prior art configuration, the peak amplitude of each electrical signal is converted to a digital number, as in Rambow, and is supplied to a computer programmed to arrange the digitized amplitude signals in a single row which corresponds to the data generated by a 360.degree. borehole scan. Each time a row is accumulated, it is transferred to a commercially available video frame buffer which stores a significant number of rows for a complete video display.
It should be noted that the borehole signal data used to create the video display is generated more slowly than the speed which is required to generate a video display. Thus, a storage buffer for the data is necessary and in the past has comprised a conventional computer memory or a commercially available frame buffer as described above.
The above-described prior art techniques for generating a video display of a borehole reflecting boundary suffer from several disadvantages. First, when a conventional computer is used to store a complete video frame, normally 512 lines, a large computer having a substantial memory is required.
A much smaller computer may be used in conjunction with a commercially available frame buffer because, in such a configuration, the computer need only have the capacity to accumulate a single row at a time which is then transferred to the frame buffer. This configuration also suffers disadvantages. When data is transferred from the computer to the frame buffer, it must be done through an input/output port. The port includes a first data bus, such typically having 8 lines to accommodate a single byte of data, which connects the computer to the port and a second data bus which connects the port to the frame buffer. A first control line is connected between the port and the computer and a second control line is connected between the port and the frame buffer. The computer must signal the port to indicate the presence of data for transfer to the frame buffer. Similarly, the frame buffer must signal the port to indicate when it is ready to receive data. Such handshake and status checking routines slow the rate at which data may be transferred.
Another disadvantage associated with the frame buffer is the necessity for generating a unique address for each byte of data transferred to the frame buffer. The video display is divided into a plurality of discrete units or pixels, each of which assumes a preselected shade of gray depending upon the value of an associated data byte. This typically requires a 17-bit address which identifies each pixel by a row and column position. For a computer which generates addresses in 8 bit numbers, three separate operations are necessary in order to produce a single 17-bit address. Since this operation must be repeated for each byte transferred to the frame buffer, the rate of operation of the system is further slowed. Slow data transfer places an upper limit on the frequency at which firing pulses can be applied to the transducer assembly and thus limits the rate at which a wellbore may be logged. When the digitized amplitude values are recorded on magnetic tape, the slow transfer of data to the frame buffer places an unacceptably low limit on the rate at which recorded data may be played back to generate a reflecting boundary image on a video monitor.
In addition to the above-described drawbacks, commercially available frame buffers are typically expensive, complex pieces of equipment which are both bulky and heavy. It is desirable to utilize compact and lightweight equipment since the same must be transported to and from remote well sites for logging operations. The above-described prior art systems necessitate the use of either a large computer or a large frame buffer.
There exists a need for a method and apparatus for generating a video display from signals produced by borehole scanning which overcomes the disadvantages associated with prior art methods and apparatus.
There exists a need for such an apparatus which is more compact and lightweight and less expensive than prior art apparatus.
There exists a need for such a method and apparatus in which a block of data may be rapidly transferred from signal data storage means, in which the data is accumulated, to a video memory means, from which data may be read for generating a video display.
There exists a need for such a method and apparatus in which a video display is generated from a borehole televiewer signal utilizing a relatively small computer without the loss of speed inherent in a commercially available frame buffer.